The Department of Justice announced an investigation Monday into the February killing of Florida teenager Trayvon Martin. George Zimmerman, a self-appointed captain of an unregistered neighborhood watch, confronted and fatally shot the teenager 70 yards from his father’s house. Do neighborhood watch programs decrease crime rates?

Possibly, but the effect is modest. A 2008 meta-analysis by researchers at the University of Glamorgan provides the most authoritative answer on this question to date, looking at 19 studies conducted in the United States, the United Kingdom, Canada, and Australia since 1977. Its authors conclude that neighborhood watch programs produce a 16-percent drop in crime, on average, but they concede research in this field is complicated. Many of the studies are anecdotal. Very few are adequately controlled or randomized, and the best-known experiment to date—one that was carried out in Minneapolis during the 1980s—showed no impact on crime whatsoever. Results can also vary tremendously between communities. As a result, it’s difficult to state with any authority whether neighborhood watch has an effect, or what that effect might be.

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Neighborhood watch has its roots in a 1960s crime surge that prompted government officials to look for inexpensive ways to bolster police effectiveness. In her 1961 book The Death and Life of Great American Cities, urban-renewal advocate Jane Jacobs argued that part of the crime problem was that people were spending more time watching television and less time sitting on their stoops or looking out their front windows. Taking a cue from Jacobs, the National Sheriffs Association launched the National Neighborhood Watch program in 1972. Coordinators in participating communities gathered their neighbors for meetings. Not all of them were interested in active patrolling; some neighborhood watches were merely intended to help people get to know their neighbors, so they could identify outsiders on community streets. (It’s important to note that George Zimmerman violated at least two long-standing rules of national neighborhood watch programs—he was armed, and he confronted Martin rather than waiting for police.)

Over the last 40 years, the apparent effectiveness of neighborhood watch has been extremely uneven. Most programs spring up in white, middle- or upper-income communities—the kind of places where crime isn’t a major problem to begin with. As a result, the programs don’t have the potential to make a significant impact, and any effect they might have on crime rates is difficult for researchers to detect. Neighborhood watches also tend to be short-lived in affluent communities, as residents stop meeting when they realize their increased vigilance is making little difference.

The program can even have a negative impact on middle-class communities. Some studies suggest that neighborhood watch makes residents feel less safe, because it constantly reminds them of the possibility of crime.

Neighborhood watch has the potential to make a difference in low-income, high-crime communities, but it rarely gets off the ground in those areas. Many residents don’t trust their neighbors, so they won’t attend or host community meetings, let alone patrol the streets at night. Distrust of police is also a factor, as local sheriffs are often the ones who provide training and support for the program. In the Minneapolis study of the 1980s, researchers found that, no matter how much effort they put into advertising neighborhood watch in high-crime communities, participation remained low.